Oximetry is an optical method for measuring oxygenated haemoglobin in blood. Oximetry is based on the ability of different forms of haemoglobin to absorb light of different wavelengths oxygenated haemoglobin (HbO2) absorbs light in the red spectrum and deoxygenated or reduced haemoglobin (RHb) absorbs light in the near-infrared spectrum. When red and infrared light is passed through a blood vessel the transmission of each wavelength is inversely proportional to the concentration of HbO2 and RHb in the blood.
Pulse oximeters can differentiate the alternating light input from arterial pulsing from the constant level contribution of the veins and other non-pulsatile elements. Only the alternating light input is selected for analysis. Pulse oximetry has been shown to be a highly accurate technique.
The contemporary pulse oximeter unit normally provides three outputs:                1. the arterial oxygen saturation        2. the heart rate        3. a fluctuating time series—the pulse oximeter trace or plethysmographic waveform        
The normal pulse oximeter waveform—the photoplethysmogram (PPG)—bears a strong resemblance to an arterial pressure waveform complete with dichrotic notch. A schematic of a typical pulse oximeter trace from a finger probe is shown in FIG. 1a. The repeating double humped (with a notch A in-between) nature of the waveform is evident in the plot. Often, the second hump disappears and a signal such as that in FIG. 1b is obtained. This may indicate a clinical condition such as reduced arterial compliance. Often, for this type of signal, there is a marked change in the gradient of the falling waveform (i.e. a kink) as indicated by the arrow B in the plot.
FIG. 2 contains a plot of three simultaneously acquired signals acquired from a patient. These are: a finger pulse oximetry trace, an ear pulse oximetry trace and an ECG. These 10 second segments have been cut from a much longer signal. Note the significant drift associated with the pulse oximetry traces.